Gas detectors and measuring devices are used to detect or measure percentages of foreign gases in the composition of industrial gases in gas tanks or gas line systems, e.g., compressed air lines.
Another field of application for gas detectors and gas measuring devices is to detect trace gases in ambient air. The quantity of gas to be analyzed is sent in a group of these gas detectors through a test tube, in which a detection reagent for the particular component to be determined is contained, the detection reaction taking place as a chromatic reaction while a change in color takes place.
A certain quantity of gas is drawn through the test tube by means of a pump driven manually or by a motor.
Such test tubes are used, for example, in the working environment of civil protection or in the maintenance of industrial plants to rule out endangerment to the rescue or maintenance teams by explosive or toxic gas components.
Bellows, diaphragm or reciprocating pumps as well as double piston diaphragm pumps are used as pumps here.
A combination of test gas tube and a controlled pump for application of a volume through a test tube is described in U.S. Pat. No. 5,295,790 A.
The combination of pressure sensors with a laminar flow element for flow measurement in a gas sampling device for determining the properties of the gas, which combination is known from U.S. Pat. No. 5,295,790 A, makes possible a linear detection of the volume flow as a function of the differential pressure dropping over the laminar flow element in the laminar range of flow in the laminar flow element. The pressure dropping over the laminar flow element is converted into an electric output signal by means of a differential pressure sensor.
This combination of a laminar flow element and a pressure sensor is a common method according to the state of the art for obtaining a linear output signal proportional to the flow. The span of the measuring range that can be obtained with such a combination of a differential pressure sensor and laminar flow element is typically in the range of 1:10 to 1:20 between the lowest measurement value that can be detected within the required measuring accuracy and the maximum detectable measured value. If the span of the measuring range shall be expanded, one typically switches over to a second measuring range with another differential pressure sensor by means of a valve. The lowest measured value and its measuring accuracy are determined by the resolution capacity and the reproducibility of the pressure sensor system and by the error effects of the gas sampling device and pressure sensor system used. The maximum detectable value is determined by both the maximum allowable pressure drop in the measuring range of the device for a reliable measurement and by the fact that laminarization by the laminar flow element is no longer effective above a certain flow rate. The consequence of a no longer effective laminarization is that the linear relationship between pressure drop and flow rate is no longer given.
An essential element concerning the measuring accuracy at the lower limit of the measuring range is the systematic measuring accuracy of the pressure sensor. Pressure sensors are subject at the attainable accuracies to the influencing variables of the environment, especially the ambient temperature and ambient pressure. Especially in a pressure sensor that detects the pressure as a measured variable by means of a deflection of a measuring diaphragm, the measurement result is affected by the ambient pressure by causing a prestress of the diaphragm, on the one hand, and, on the other hand, by the ambient temperature due to thermal expansion of the diaphragm.
DE 10131688 B4 describes a pressure sensor, in which the effect of ambient temperature is compensated by selecting a suitable combination of materials for the measuring diaphragm. The effect of ambient pressure on the measurement result cannot be compensated by means of a suitable combination of materials as a design feature.
DE 2823315 describes a control of a tube pump with a constant pressure drop over the test tube as a controlled variable.
DE 19 825103 A1 describes a volume control of a test tube pump, wherein a differential pressure sensor is used in combination with a flow resistance for controlling the volume.
A laminar flow element (LFE) represents a flow resistance in such an application, with the peculiarity that the flow is laminarized in a defined flow range.
The effect of ambient pressure on the determination of the volume being delivered through the test tube is not taken into account in DE 19 825103 A1.
It is necessary for the universal applicability of the tube pump for various types of gas detector tubes that the quantity delivered by the tube pump be adaptable to the different types of gas detector tubes. It is advantageous for this that the tube pump draw the quantity of gas to be measured through the test tube under a constant vacuum of, for example, 100 mbar, as it is described in U.S. Pat. No. 5,295,790 A.
Depending on the type of tube and flow resistance thereof, a flow rate of 50-60 mL/min to 2-3 L/min is established. The flow rate detected by means of the differential pressure sensor in combination with a laminar flow element is integrated into a gas volume and compared with the desired volume specific of the type of tube. The pump is switched off when the desired volume is reached.
It is absolutely necessary for a subsequent colorimetric analysis and the measuring accuracy that can be achieved overall that the desired volume of the quantity of gas drawn through the test gas tube from the measuring environment is maintained highly accurately. The difficulty in obtaining a flow rate accurately, reproducibly and with a constant relative accuracy in the range of <5% over a broad span of measuring range with a single combination of differential pressure sensor and laminar flow element to guarantee the requirements imposed on maintaining the desired volumes specific of the different types of tubes arises in conjunction with the broad span of measuring range required by the great variety of tube types. It is necessary for this that the effect of ambient pressure on the measuring accuracy of the differential pressure sensor be likewise compensated besides the compensation of the effect of ambient temperature on the measuring accuracy of the differential pressure sensor.